The objectives of this project/program can be summarized as follows: 1. To undertake a multidisciplinary study to elucidate the mechanisms of carcinogenesis of the mammary gland in the rat. 2. To study the biochemical mechanism of the action of three major hormones; prolactin, estrogen, and progesterone on the mammary gland during normal physiological process and during the process of carcinogenesis. 3. To investigate the inter-relationship between estrogen carcinogenesis and polycyclic hydrocarbon carcinogenesis of the mammary gland. 4. To elucidate the morphogenetic and host factors which contribute to the development of breast cancer. 5. To carry out biochemical studies to elucidate the nature of hormone-dependency of breast cancer in humans. 6. To compare data obtained from studies in the experimental tumor model with results of human tumor studies.